Technical Field
The present disclosure relates to a micro-electro-mechanical, in the following MEMS (Micro-Electro-Mechanical System), force sensor of a capacitive type and to a corresponding force sensing method.
Description of the Related Art
The increasing use is known of sensors including micromechanical sensing structures, made at least in part of semiconductor materials and using MEMS technology, for example in portable electronic apparatuses, such as tablets, smartphones, digital audio players, photographic or video cameras and consoles for videogames.
MEMS sensors have a number of advantageous features, amongst which extremely compact dimensions, reduced consumption levels and good electrical performance and may be used, for example, for the provision of UIs (User Interfaces) for portable electronic apparatuses.
A MEMS sensor generally comprises: a micromechanical sensing structure, designed to transduce a mechanical quantity to be detected (for example, a force) into an electrical quantity (for example, a capacitive variation, in the case of capacitive sensing structures); and an electronic reading circuit, usually integrated as an ASIC (Application-Specific Integrated Circuit), designed to carry out appropriate processing operations (amongst which operations of amplification and filtering) on the transduced electrical quantity, in order to supply an electrical output signal, whether analog (for example, a voltage), or digital. This electrical signal is made available for an external electronic apparatus (the so-called “host”) incorporating the MEMS sensor; for example, it is received at input by a microprocessor control unit of the electronic apparatus.
The micromechanical sensing structure of a MEMS sensor of a capacitive type generally comprises a mobile electrode, made as a diaphragm or membrane, set facing a substantially fixed electrode, being separated from the latter by an air gap (or separation region). The mobile electrode is generally anchored elastically, by a perimetral portion thereof, to a substrate, while a central portion thereof is free to move or bend in response to the quantity to be detected (for example, a force acting on the mobile electrode). The mobile electrode and the fixed electrode form the plates of a sensing capacitor and bending of the membrane that constitutes the mobile electrode causes a variation of capacitance of the sensing capacitor, due to the variation of the height or thickness of the air gap separating the electrodes.
Known solutions for MEMS force sensors of a capacitive type have certain disadvantages, linked in particular to a reduced detection sensitivity for low values of the applied force.
In this regard, FIG. 1 shows a typical plot of the sensing capacitance, designated by C, as a function of the applied force, designated by F, in a MEMS force sensor of a known type (the relation capacitance vs. applied force refers in this case to a simple system with a capacitor with plane and parallel faces). The plot shows a low value of sensitivity in a wide range of force values, in the example between 0 and approximately 500 gram-force (gf).